The present invention relates to autothermal reformers and methods of operating such reformers.
The search for alternative power sources has focused attention on the use of electrochemical fuel cells to generate electrical power. Unlike conventional fossil fuel power sources, fuel cells are capable of generating electrical power from a fuel stream and an oxidant stream without producing substantial amounts of undesirable by-products, such as sulfur oxides, nitrogen oxides and carbon monoxide. However, the commercial viability of fuel cell systems will benefit from the ability to efficiently and cleanly convert conventional hydrocarbon fuel sources, such as, for example, gasoline, diesel, natural gas, ethane, butane, light distillates, dimethyl ether, methanol, ethanol, propane, naphtha, kerosene, and combinations thereof, to a hydrogen-rich gas stream with increased reliability and decreased cost. The conversion of such fuel sources to a hydrogen-rich gas stream is also important for other industrial processes, as well. Several technologies are available for converting such fuels to hydrogen-rich gas streams.
Steam reformers convert hydrocarbon fuels to hydrogen-rich reformate gas streams. Fuel and steam are reacted in reactors filled with catalyst (typically nickel-, copper- or noble metal-based), and primarily hydrogen, carbon dioxide (CO2), and carbon monoxide (CO) are produced. For example, the following principal reactions occur in the steam reforming of methane (and natural gas):                                                                                           CH                  4                                +                                                      H                    2                                    ⁢                  O                                                                    ⇌                                                      CO                +                                  3                  ⁢                                      H                    2                                                                                                                          CO                +                                                      H                    2                                    ⁢                  O                                                                    ⇌                                                                        CO                  2                                +                                  H                  2                                                                                                                          CH                  4                                +                                  2                  ⁢                                      H                    2                                    ⁢                  O                                                                    ⇌                                                                        CO                  2                                +                                  4                  ⁢                                      H                    2                                                                                      "AutoLeftMatch"                            (        I        )            
The overall reaction (I) is highly endothermic, and is normally carried out at elevated catalyst temperatures in the range of about 650xc2x0 C. to about 875xc2x0 C. Such elevated temperatures are typically generated by the heat of combustion from a burner incorporated in the steam reformer.
Steam reforming is generally unsuitable for heavier fuels and is normally limited to paraffinic naphtha and lighter fuels. In addition, nickel-based steam reforming catalysts are easily poisoned by sulfur in the fuel. As a result, if the fuel to be reformed contains sulfur, the reformer is usually preceded upstream by hydrotreating apparatus such as a hydrodesulfurizer (HDS) and an H2S removal device, such as a metal oxide absorbent bed, in order to remove or reduce to extremely low levels any sulfur present in the fuel. This tends to increase the cost and complexity of the overall fuel processing system. Thus, steam reformers are generally unsuitable for heavier and/or sulfur-laden fuels, such as gasoline or diesel, for example.
Autothermal reforming is an approach that combines catalytic partial oxidation and steam reforming. Partial oxidation employs substoichiometric combustion to achieve the temperatures to reform the fuel. Fuel, oxidant (oxygen or air, for example), and steam are reacted to form primarily hydrogen, CO2 and CO. An advantage of autothermal reforming technology is that the exothermic combustion reactions are used to drive the endothermic reforming reaction (I).
Autothermal reformers typically employ noble metal catalyst beds operating at temperatures of from about 870xc2x0 C. to about 1300xc2x0 C. In comparison to steam reformers, an advantage of autothermal reformers is that at these high operating temperatures, sulfur in the fuel, which is present primarily as H2S, does not significantly poison the catalyst and permits downstream sulfur removal. This may result in a simpler fuel processing system, as an HDS is not required. A further advantage of autothermal reformers is that start-up times also tend to be shorter due to the heat supplied to the catalyst bed by catalytic combustion of the fuel. Yet another advantage of autothermal reformers is that they are capable of reforming heavier fuels than steam reformers. Thus, autothermal reformers may be more desirable for use in fuel processing systems employing heavier and/or sulfur-laden fuels, such as gasoline or diesel fuel, for example.
However, conventional autothermal reformers have at least two major disadvantages, particularly with respect to fuel cell-related applications. First, conventional autothermal reformers and associated heat recovery equipment tend to be quite large, which impacts material costs and overall manufactured cost. This is especially disadvantageous in vehicular applications, where space is also at a premium. Second, it can be difficult to adequately vaporize fuels such as diesel, for example, and distribute the fuel uniformly within the catalyst bed employing conventional autothermal reformers. These disadvantages tend to be compounded in vehicular applications employing diesel, for example, as the fuel.
It is desirable to have a rugged, compact autothermal reformer capable of efficiently reforming difficult fuels, such as diesel.
An autothermal reformer for converting a fuel into a reformate stream comprising hydrogen is presented. The present reformer comprises:
(a) a closed vessel, the vessel having a top end and a bottom end, the vessel comprising at least one insulation layer adjacent the interior surface of the vessel;
(b) a first reactant manifold disposed within the vessel for receiving and distributing a first reactant stream, the first reactant manifold having a plurality of mixer tubes extending therefrom, each of the mixer tubes having an inlet end and an outlet end, the injection tubes disposed in a separator member; and
(c) a second reactant manifold disposed within the vessel for receiving and distributing a second reactant stream, the second reactant manifold comprising a plurality of injection tubes, each of the injection tubes having an inlet end and an outlet end, the injection tubes extending through the first reactant manifold and fluidly isolated therefrom;
wherein the outlet end of each of the plurality of injection tubes extends into the inlet end of one of the mixer tubes, forming a gap between the outer wall of the injection tube and the inner wall of the mixer tube.
Preferably, the gap between the mixer tubes and corresponding mixer tubes is an annular gap.
In one embodiment of the present reformer, the first reactant stream comprises substantially vaporized fuel and the second reactant stream comprises oxidant.
In a preferred embodiment, the first reactant stream comprises oxidant and the second reactant stream comprises substantially vaporized fuel.
The injection tubes and mixer tubes are arranged so as to uniformly mix and distribute the reactant stream within the present reformer. For example, the injection tubes and mixer tubes may be arranged in a hexagonal array.
The mixer tubes may comprise openings in the separator member and cooperating openings in one end of the first reactant manifold. Preferably, the first reactant manifold and mixer tubes form a shell-and-tube assembly. The length of the mixer tubes may be at least ten times the inner diameter of the mixer tubes.
The separator member that the mixer tubes are disposed comprises insulating material, preferably a ceramic composition.
The present reformer further comprises a reforming section disposed within the vessel, the reforming section comprising a combustion and gasification catalyst bed spaced apart from and in fluid communication with the reactant mixer layer, and a steam reforming catalyst bed in contact with the combustion and gasification catalyst bed. The cooperating surfaces of the reactant mixer layer and the combustion and gasification catalyst bed form a plenum therebetween. The combustion and gasification catalyst bed preferably comprises at least one monolith comprising noble metal catalyst components disposed on a porous support, where the support is preferably a ceramic honeycomb. Similarly, the steam reforming catalyst bed preferably comprises at least one monolith comprising noble metal catalyst components disposed on a porous support, where the support is preferably a ceramic honeycomb. The plenum may also be at least partially filled with a particulate combustion and gasification catalyst, preferably comprising noble metal catalyst components disposed on a pelletized porous support.
Another embodiment of the present reformer further comprises a first heat exchange member disposed within the vessel and in thermal communication with the reformate, the first heat exchange member for receiving a steam stream and superheating the stream therein by heat exchange with the reformate. The first heat exchange member may comprise a helical coil, for example. Preferably, the first heat exchange member comprises a bare helical coil disposed within a high temperature alloy steel metal annulus packed with high-temperature metal heat transfer packing, such as high void fraction pall rings or saddles, for example.
Yet another embodiment of the present reformer further comprises a second heat exchange member disposed within the vessel and in thermal communication with the reformate, the second heat exchange member for receiving a feed stream comprising water and vaporizing the feed stream therein by heat exchange with the reformate to produce a steam stream. The heat exchange member may comprise a helical coil, for example. Preferably, the helical coil comprises a finned tube helical coil disposed within an alloy steel sheet metal annulus.
Still another embodiment of the present reformer further comprises the first heat exchange member and the second heat exchange member, as described.
In another embodiment, the present reformer further comprises a fuel vaporizer for substantially vaporizing the fuel in the presence of superheated steam, the fuel vaporizer disposed within the vessel and comprising a helical coil. In further embodiments, the present reformer comprises the fuel vaporizer and the first heat exchange member, or the fuel vaporizer and the second heat exchange member. In another preferred embodiment, the present reformer comprises the fuel vaporizer, the first heat exchange member, and the second heat exchange member.
In a further embodiment, the present reformer further comprises the first and second heat exchange members, and external heat exchange members associated with the external surface of the vessel for flowing a heat exchange fluid therethrough. In yet another preferred embodiment, the present reformer further comprises the fuel vaporizer. Either embodiment may further comprise an external insulating layer adjacent the exterior surface of the vessel.
The external heat exchange members may comprise plate coil or helical coil, for example. Preferably, the heat exchange fluid comprises the feed stream.
Methods of operating the present autothermal reformer are also provided.
In one method, the reformer comprises a closed vessel, the vessel having a top end and a bottom end, a reforming section disposed within the vessel for catalytically converting a reactant stream comprising fuel to a reformate stream comprising hydrogen, and a fuel vaporizer disposed within the vessel, the fuel vaporizer comprising a helical coil. The method comprises:
(a) supplying a liquid fuel and a superheated steam stream to the fuel vaporizer;
(b) substantially vaporizing the fuel in the fuel vaporizer to produce a vaporized fuel stream;
(b) mixing the vaporized fuel stream with an oxidant stream to produce a reactant stream; and
(d) supplying the reactant stream to the reforming section.
The superheated steam stream is preferably supplied to the fuel vaporizer at a temperature of about 800xc2x0 C. Where the liquid fuel comprises diesel, the diesel may be supplied to the fuel vaporizer at a temperature of about 300xc2x0 C., if desired. Preferably, where the liquid fuel comprises diesel, the vaporized fuel stream exits the fuel vaporizer at a temperature of at least about 425xc2x0 C.
In another method of operating the present autothermal reformer, the reformer comprises: a closed vessel, the vessel having a top end and a bottom end, the vessel comprising a product outlet and at least one insulation layer adjacent the interior surface of the vessel; a reforming section disposed within the vessel for catalytically converting a reactant stream comprising fuel to a reformate stream comprising hydrogen; a first heat exchange member disposed within the vessel, the first heat exchange member in thermal communication with the reformate; and, a second heat exchange member disposed within the vessel and fluidly connected to the first heat exchange member and in thermal communication with the reformate. The method comprises:
(a) supplying the reactant stream to the reforming section and producing a reformate stream;
(b) supplying a feed stream comprising water to the second heat exchange member;
(c) flowing the reformate stream in heat exchange relation with the second heat exchange member to produce a steam stream therein;
(d) supplying the steam stream to the first heat exchange member;
(e) flowing the reformate stream in heat exchange relation with the first heat exchange member to produce a superheated steam stream therein; and
(f) directing the reformate stream out of the vessel through the product outlet,
wherein the temperature of the reformate stream is moderated by heat exchange such that the temperature of the reformate stream in direct contact with the insulation layer is no greater than about 650xc2x0 C. preferably no greater than about 350xc2x0 C.
The reformer may further comprise external heat exchange members associated with the external surface of the vessel for flowing a heat exchange fluid therethrough, and the method may further comprise flowing the heat exchange fluid through the external heat exchange members and maintaining the temperature of the vessel at a temperature below the operating temperature of the reformer. Preferably, temperature of the vessel is also maintained at a temperature above the internal gas dew point of the reformate stream. For example the temperature of the vessel may be maintained at a temperature of at least about 230xc2x0 C. Preferably, the heat exchange fluid comprises the feed stream.